1. Field of the Invention
The present invention relates to a sleeve for covering cable joints and which are made of compounds of cross-linked polymeric material, to be applied to several different cables having different outer diameters.
2. Prior Art Description
In order to provide a junction between electrical cable lengths which carry electrical energy, particularly in the field of medium and high voltages, the layers around the cable conductors are stripped off stepwise at the ends, that is, the conductors are devoid of the respective insulating layers and, if present, semiconductive layers, for the purpose of exposing the conductors thereby allowing their mutual connection and subsequently, the zone without the insulating layer is filled with appropriate materials and then covered with an outer layer or sleeve so as to restore the required insulating characteristics in the junction area.
For such purposes, a tubular element, hereinafter referred to as a sleeve, is fitted over the junction area. The sleeve is made of cross-linked polymeric material consisting of several layers each having specific electrical features, and as a whole, it is designed to be elastically clamped around the surface of the insulating layer of the connected cables.
The sleeve is radially expanded and maintained under expanded conditions until the moment of use. After the sleeve has been put over the junction area its shrinkage around the surface of the cable insulating layer is carried out thereby exerting a pressure thereon capable of ensuring the necessary electrical requirements.
In order to keep the sleeve under expanded conditions, it can be made of thermoshrinkable material, a material which is capable of maintaining the expansion it has received until it is shrunk by the application of heat. However, this technique requires delicate operations for the installation of the sleeve because heating means such as open flames are needed to carry out the thermoshrinkage of the sleeve.
Alternatively, the sleeve may be made of an elastic material and fitted around a tubular support body which is removed by known techniques after the sleeve has been brought to the intended position which enables the sleeve to elastically shrink and clamp the cable insulating layer.
For such purposes, elastic multilayered sleeves have been made, each layer being of small thickness and being elastically expanded and independently supported on respective tubular bodies, such layer being applied one after the other until the intended thickness has been reached. In this way, each layer is submitted to the minimum permissible expansion, but several contact areas are created between the different separate layers which increases the risk that polluting substances or air bubbles may be incorporated. Consequently, the phenomena of partial discharges may arise which cause a joint failure in a short time.
Also, sleeves of great thickness have been made as disclosed, for example, in U.S. application Ser. No. 464,370, filed Jan. 12, 1990, assigned to the assignee of the present application and entitled "Multi-Layer Elastic Sleeves for Electric Power Cable Joints and Joints Therewith, in which in particular provision is made for a sleeve of a single size for covering cable junctions having different sizes by the use of sleeves disposed in the elastically expanded state on the respective supports at the factory and kept as such until the moment of their installation.
With the sleeve of said patent application, the problem of imparting to the sleeve an expansion sufficient to enable it to be fitted over the cable having the largest diameter in the group of the intended sizes while at the same time enabling the sleeve to be efficiently clamped also around the cables of the smallest size in the group has been solved by the use, for the outermost layer of the sleeve, of a material having a reduced residual deformation when the applied expansion stress has ceased. Therefore, the sleeve is adapted to allow the whole sleeve to be clamped around the small-sized cables in an efficient manner.
In fact, the polymeric materials to be used for the manufacture of sleeves in which each layer needs particular electrical features, well known in the field, generally have an elastic return to the original size which is incomplete, that is, the sleeve has a temporary residual deformation the degree of which depends, among other things, on the value of the previously imposed deformation and the temperature and time of stay in the stretched state. Said residual deformation decreases as time goes by and tends to become zero after a certain lapse of time, in the range of some days or months at room temperature (.ltoreq.30.degree. C.).
Due to the fact that after the sleeve has been fitted on the cable junction, it is impossible to wait, before putting the cable itself into service, for a time sufficient to achieve complete size recovery which is necessary for the correct clamping of the sleeve around the cable. For the manufacture of sleeves expanded on a support at the factory of the type set forth in said patent application, the use of materials having particular properties of reduced residual deformation and capable of causing the shrinkage of the whole sleeve has been proposed for the outermost layer of the sleeve.
The materials of the above type, however, are of difficult formulation because it is difficult to associate the mechanical characteristics required of them with the necessary electrical properties, and therefore, in accordance with said patent application, it is solely the outer layer which must be sized so as to cause the elastic shrinkage of the whole sleeve. Therefore, there is a demand for a sleeve adapted to be used for the purpose, which exhibits identical elastic characteristics in the different layers, without requiring the use of materials having the above stated high elastic properties.